This invention relates generally to radial keratotomy, and more particularly to optical zone markers in conjunction with such eye cornea surgical procedures.
Radial keratotomy is a relatively new surgical procedure for reducing myopia. Incisions are made in the cornea radially extending from the optical clear zone which tend to flatten the corneal surface, thereby reducing near sightedness and the cornea's optical power. Astigmatism may also be reduced in a similar procedure by placing the incisions in such a way that the cornea will flatten along one axis only. Although this procedure is widely practiced, long term effectiveness and safety have not been established.
Properly identifying and marking the opticl center of the visual axis of the patient's eye is a most important step in the preparation of the eye for surgery. If the optically clear zone to be later marked is only slightly decentered, the end of the radial incisions later made may encroach on the visual axis and increase the potential for problems of glare and astigmatism may also be induced. Prior to the marking of the center of the visual axis, the visual axis of the eye must be determined utilizing one of several well-known techniques. After the mark is accurately placed onto the patient's eye prior to surgery, an optical zone marker, properly sized to equal the diameter of the optically clear zone, is used by pressing it into the cornea, centered around the optical center mark, to cause a temporary circular indentation in the cornea to guide the placement of incisions.
To reiterate, radial keratotomy involves the making of incisions into the cornea radially extending from the optical center of the eye. However, these incisions do not extend to the optical center, but rather extend from a circle having its center at the optical center of the eye. The diameter of the circle from which the incisions radially extend varies depending on the scope of the corrective surgery required and the diameter of the patient's optical clear zone. Impinging on that optical clear zone by incision is one of the primary causes of post surgical glare problems.
Properly sized optical zone markers, then, are designed to be pressed onto the cornea around the optical center mark. These markers, which are primarily circular, but may also be oval, usually have a device such as a needle tip or cross hair to indicate the exact center of the circular marking edge for precise alignment on the cornea. The original tube-like optical zone markers in variously modified forms have been designed by Drs. Fyodorov, Hoffer, Berkeley and Thornton.
A primary problem with conventional optical zone markers results from binocular parallax as a result of the thickness of the outer tubular portion in relation to the cross hair or needle tip identifying its center. Again, serious degradation of the patient's eye may result if the optical zone mark imposed on the cornea is not accurately positioned in relation to the optical center of the eye.
An early technique for reducing this binocular parallax problem was achieved by reducing the length of the marker portion of optical zone markers, which have now been reduced down to about one millimeter long. Such markers are available from Storz, model number E-9030 in various diameters. It should be noted that this dimensional change only reduces, but does not eliminate, parallax error. Additionally, the cross hairs of some such devices have been known to inadvertently contact the cornea because the device was too short.
Nonetheless, because many optical zone markers continue to incorporate fine wire cross hairs to identify the center of the circular marker tube, the parallax problem remains as an element of inaccurate placing of the optical zone mark with such devices. These currently available instruments, having either cross hairs, needle point, or a centered ring within the generally circular marker tube to assist in aligning the instrument with the optical eye center and providing only one point in the vertical plane to be used in aligning the optical zone marker directly above the optical center, must be used extremely carefully while nonetheless running the risk of misalignment.
An additional problem associated with currently available optical zone markers resides in the user's requirement to have a wide range of sizes of such markers available from three millimeters to eight millimeters in diameter in one quarter millimeter increments. Because the currently available instruments are made of surgical stainless steel and are extremely expensive to manufacture, a complete set of such optical zone markers is extremely expensive.
Surgical stainless steel optical zone markers which include attached stainless steel cross hairs, include the inherent additional limitation that, after repeated use, the center sighting means may become loosened from the cylindrical tubular portion and have been known to fall into the patient's eye during surgery or be found inoperative when required.
A further problem resides in the fact that, as conventional optical zone markers are lowered close to the cornea's surface, the tubular portion of the marker, made of surgical steel, blocks out the available light striking the center portion of the cornea, thus reducing the visibility of the corneal surface. This further increases the difficulty of proper orientation prior to indenting the cornea.
The present invention provides features which overcome all of the above limitations. The parallax problem is virtually eliminated by the structure of the present invention; the cost of manufacturing a utilized injection molded part is insignificant and may be manufactured of various sizes inexpensively; the marker may be molded of clear material to reduce the light-obliterating limitation of surgical steel markers; manufacturing tolerances are at least as accurate, if not more so, than those found in conventional surgical stainless steel markers; the cross hairs, because integrally molded, virtually eliminate the possibility of these cross members falling into the patient's eye.